Image: A gold-gold collision at maximum energy as seen by the Phobos detector. Phobos consists of a cylindrical array of silicon detectors and two spectrometer arms surrounding the interaction region where the gold nuclei collide. Colored dots show the locations where silicon was struck by the thousands of produced particles. The red lines are reconstructed trajectories of some of those particles. Image courtesy of RHIC.
“It's being debated,� said Argonne physicist Birger Back, a member of the Phobos Detector team at RHIC. “Did we actually find what we set out to find, which is the quark-gluon plasma? In some sense, we haven't. We found something different. We found that nature behaves different than what we — perhaps naively — expected.�
RHIC as a racetrack
RHIC accelerates two beams of ions — atoms stripped of their electrons — giving them a strong positive electric charge to take full advantage of the machine's accelerating and bending capabilities. RHIC primarily uses ions of gold, one of the heaviest common elements, because its nucleus is densely packed with particles. The beams travel in opposite directions around a 2.4-mile, two-lane “racetrack� at 99.995 percent of the speed of light. Objects nearing the speed of light gain mass via relativistic effects (hence the machine's name), so when the beams intersect, the colliding ions achieve fantastic temperatures and densit ies: more than a trillion Kelvins and 10 times the density of normal nuclear matter.
These extreme conditions were expected to have liberated quarks and gluons from their quantum prisons inside the gold protons and neutrons, producing a primordial particle soup — quark-gluon plasma.
Argonne's STARring role
Scientists from Argonne's Physics and High Energy Physics divisions were involved in two of the four RHIC experiments: STAR and Phobos.
The STAR detector is a large volume, gas-filled detector to record charged particle tracks in a solenoidal magnetic field. This is surrounded by electromagnetic calorimeters, which were partially designed and constructed at Argonne . The calorimeters can also detect high energy photons and neutral particles.
High-energy physicists Bob Cadman, Dave Underwood and Hal Spinka analyzed proton-proton collisions and gold-on-gold collisions at STAR, which contributed to the observation of qualitative differences in particle production at RHIC
The Argonne high-energy physicists are primarily interested in the ways quarks and gluons contribute to the total “spin� — or intrinsic angular momentum — of protons. They study collisions of beams of polarized protons, which are produced in such a way that their spins are aligned.
The Phobos detector measures the temperature, size and density of the fireball produced in the heavy-ion collisions. It also reveals the ratios of the particles produced.
Phobos consists of silicon detectors surrounding the interaction region, which were designed, tested and assembled in a joint effort by scientists and engineers at Argonne and the University of Illinois at Chicago . FermiLab's Silicon Detector Laboratory performed microwire bonding to connect the detectors to the microchip pre-amplifiers. Massachusetts Institute of Technology developed two high-quality magnetic spectrometers that study 1 percent of the produced particles in detail.
Surprising interactions
“The original expectation was that quarks and gluons would move freely inside the plasma with very little interaction,� Back said. “The surprising fact is that there is very strong interaction among the particles. We've seen several signals to verify that.�
A key signal that lets physicists know when two quarks collide is the production of a pair of pions, which leave the scene of the collision in exactly opposite directions perpendicular to the beam.
In the high-energy gold-gold experiment, in which the colliding particles each contained 79 protons and 118 neutrons, some of those pions went missing — a key clue.
“When gold protons collided near the edge of the interaction, one pion could escape in the usual way,� Cadman said. “The other disappeared, which indicated that the matter produced in the collision had abnormally high density.�
Rather than behaving like a gas, as was expected, the collisions produced a state of matter more like a liquid. In fact, it behaved like an “ideal liquid,� with extremely low viscosity.
“From the emission pattern of the particles coming from the collisions, we see what we call a ‘collective flow.' That was a surprise,� Back said. “That was one thing that our experiment, Phobos, contributed to.�
In a gas, particles can be emitted in all directions; in a denser medium, such as liquid, the particles are emitted preferentially. The matter produced in the RHIC collisions occurred in an almond-shaped region; more particles were emitted along the thin axis than the long axis, suggesting that particles emitted in that direction were being scattered or reabsorbed.
“I think everyone agrees that we have created matter with an energy density that far exceeds what is needed for a quark-gluon plasma,� Back said. “However, it's circumstantial evidence: all the conditions are right, but it doesn't exactly prove that you have plasma. It didn't behave as it was supposed to. There were a number of signals predicted, but they have not been found. There's no smoking gun.�
At higher energies, the liquid may become non-interacting — more like the quark-gluon plasma that was expected, Back said. RHIC researchers are now analyzing the results of copper-copper collisions at various energies. Results may be released in August.
“A lot is up to the theorists to interpret what we've seen,� Back said. “There are years of analysis and paper-writing ahead.�
The experiments have allowed physicists to probe the theories of quark behavior. The theory, called
quantum chromodynamics, is too difficult to calculate even on today's most powerful supercomputers. Data from these experiments will allow physicists to improve their models and their ability to calculate the interactions of quarks and gluons.
“It's nice when you understand what's going on, but it's more exciting when you find something you don't understand,� Back said.
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Source: Argonne National Laboratory
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